1. Field of the Invention
The present invention relates to switching power supplies of synchronous-rectification LLC resonant converters.
2. Description of the Related Art
Conventionally, switching power supply devices including various types of synchronous-rectification LLC resonant converters have been designed.
FIG. 1 is a circuit diagram of a switching power supply device described in Japanese Unexamined Patent Application Publication No. 2007-274789. The switching power supply device described in Japanese Unexamined Patent Application Publication No. 2007-274789 is an LLC resonant converter. As illustrated in FIG. 1, in the switching power supply described in Japanese Unexamined Patent Application Publication No. 2007-274789, a current transformer is connected in series to an inductor including a primary winding of a transformer, a resonant inductor, and a resonant capacitor. Current flowing to a resonant circuit, that is, current flowing to the primary winding, is detected by the current transformer. The detected current is input to a drive circuit, and the drive circuit controls turning on and off of a secondary-side switching element (synchronization rectification element), on the basis of the detected current.
FIG. 2 is a circuit diagram illustrating a switching power supply device described in Japanese Registered Utility Model No. 3126122. The switching power supply device described in Japanese Registered Utility Model No. 3126122 is a synchronous-rectification LLC resonant converter of a half bridge type. As illustrated in FIG. 2, in the switching power supply device described in Japanese Registered Utility Model No. 3126122, a control circuit is provided on the secondary side. The control circuit controls turning on and off of a primary-side switching element as well as a secondary-side switching element. At this time, after electrical conduction of the primary-side switching element is achieved, the control device allows electrical conduction of the secondary-side switching element when a predetermined time interval (for example, 0.4 microseconds) has passed. Furthermore, after disconnection of the primary-side switching element is achieved, the control device allows disconnection of the secondary-side switching element when a predetermined time interval (for example, 0.15 microseconds) has passed. That is, the control circuit of the switching power supply device described in Japanese Registered Utility Model No. 3126122 controls turning on and off of the primary-side switching element and the secondary-side switching element (synchronization rectification element), with a predetermined time interval between the control for the primary-side switching element and the control for the secondary-side switching element.
As described above, the switching power supply devices described in Japanese Unexamined Patent Application Publication No. 2007-274789 and Japanese Registered Utility Model No. 3126122 control turning on and off of a secondary-side switching element (synchronous rectification element) using different methods. However, in the switching power supply device described in Japanese Unexamined Patent Application Publication No. 2007-274789, in order to drive the secondary-side switching element (synchronous rectification element), a current transformer that detects current of a resonant circuit, that is, current flowing to a primary winding, needs to be provided, thus increasing the number of component elements of the switching power supply device. Furthermore, in order to generate and supply a driving signal for the synchronous rectification element, a high-accuracy, high-speed comparator needs to be provided. Accordingly, the configuration of the switching power supply device becomes complicated, thus generating a problem such as increase in the cost.
Furthermore, as described in Japanese Registered Utility Model No. 3126122, in the configuration in which turning on and off of a primary-side switching element and a secondary-side switching element (synchronous rectification element) is controlled, with a predetermined time interval between the control for the primary-side switching element and the control for the secondary-side switching element, in the case where a switching frequency is lower than the resonant frequency of a resonant circuit, negative current flows when the secondary-side switching element (synchronous rectification element) is turned on. Thus, reverse current toward the primary side may be generated.